Caption: Calculating friction forces. Animation of a mass (red block) on a slope, with diagrams illustrating how the friction force that keeps the block stationary is calculated. The angle of the slope is labelled theta. Arrows label the forces in balance here. The weight or gravitational force (blue arrow) is a downwards force from the centre of mass. The reaction force (orange arrow) acts outwards and perpendicular to the slope. The friction force (red arrow) acts parallel to the slope. This can be rearranged as a right-angled triangle (end of clip) and the mathematical principles of trigonometry applied to calculate the values of the reaction and friction forces for a known weight. The reaction force is the weight multiplied by the cosine of the angle theta. The friction force is the weight multiplied by the sine of the angle theta.